The present invention relates in general to safety devices for stopping elevator cars and, in particular, to such safety devices actuated by a speed governor mounted on the elevator car.
The U.S. Pat. No. 4,662,481 discloses a safety device having a speed governor mounted on an elevator car, which device prevents excessive speed of the elevator car during downward travel. The speed governor is essentially a spring cylinder pivotally supported at one end on the elevator car and attached at the other end to a fork shaped frame, which frame carries external rotatable and internal fixed concentric cylinders. The frame is also connected with the safety device by an actuating rod. The rotatable cylinder has a peripheral groove corresponding in cross section to the profile of the guide rail. The spring cylinder biases the roller cylinder to engage the groove with the guide rail so that travel by the elevator car rotates the rotatable cylinder. A release mechanism with centrifugal weights is positioned in the interior of the rotatable cylinder to monitor the speed of the car. In the case of excessive speed of the car in the downward direction, the release mechanism locks the rotatable cylinder to the fixed cylinder. Friction between the guide rail and the locked rotatable cylinder causes an upward relative motion about the pivot point of the spring cylinder thereby triggering actuation of the safety device through the actuating rod. The spring cylinder is blocked from pivoting when the car is travelling in the upward direction. This safety device is to a great extent similar in operation to the prior art safety devices triggered by a limiter rope.
The release mechanism positioned in the interior of the rotatable cylinder has a pair of weights tensioned initially by a spring, which weights rotate with the rotatable cylinder. As the weights move outwardly, due to the centrifugal forces generated by rotation of the cylinder, a hook shaped end on each weight engages teeth formed on the internal fixed cylinder.
A drawback of the above described safety device is the complicated mechanical construction of the release mechanism positioned inside the rotatable cylinder. High manufacturing and assembly costs connected therewith increase the cost of the speed governor. A further drawback of this safety device is the large mass to be moved in the case of breakdown. Furthermore, the speed governor can be relied upon only if sufficient friction exists between the guide rail and the rotatable cylinder, which condition requires periodic cleaning of the guide rails.